A shortage of oxygen and rising temperatures underwater caused hundreds of fish to wash up dead on Lake Elsinore’s shore.
The incident, which is now over, is similar to what happened in August 2015 and August 2017, when die-offs occurred over several days when temperatures exceeded 100 degrees.
The dead fish were Threadfin shad, which are sensitive to temperature and the amount of oxygen in the water, said Adam Gufarotti, community support manager for the city of Lake Elsinore.
The sight surprised lake-goers such as Lake Elsinore’s Bryan Rose, who took a father-son fishing trip there Friday, Sept. 13.
They immediately saw hundreds of dead fish scattered along the beach’s sandy perimeter.
“‘Why did they die, Dad?’” Rose recalled his son, Santiago Rose, asking.
The city began cleaning up the corpses Friday, Sept. 13, a day after officials believe the die-off likely began.
Though several factors contribute to a fish die-off, Gufarotti said one important reason was the lack of oxygen in the water.
The lake’s dissolved oxygen levels decreased for a variety of reasons, he said, but major ones were the warmer water temperatures, which were fueled by the recent heat wave and the Airport fire that started in Orange County but reached areas near Lake Elsinore, Gufarotti said.
As water heats up, its ability to hold oxygen decreases, he said, and that left the shad stressed and unable to take in oxygen.
While the shad didn’t survive, larger fish in the lake that can withstand higher temperatures and are less sensitive to oxygen changes were not affected, he said.
Algae also contributed to the shad die-off.
Gufarotti said that, in lakes, algae has a big impact on the oxygen balance.
Though the theory has not yet been tested, officials believe that “there was so much ash on the lake from the fires that it created a layer of ash on the surface.”
As a result, during the day, the algae did not produce as much oxygen through photosynthesis because the sunlight was not penetrating the top layer of the lake, he said.
What is the city of Lake Elsinore doing about the deaths?
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City workers used shovels and rakes to lift the hundreds of fish carcasses into trucks Friday, Sept. 13, and Saturday, Sept. 14, Gufarotti said.
The lake already has a nanobubble system, a technology that uses tiny bubbles to improve water quality and remove algae, but Gufarotti said two more machines will be installed in October, with the first scheduled for installation Oct 7.
The die-off did not endanger the safety of swimmers or people and the lake is open for boating and swimming.
For information on lake testing and water-quality monitoring, click here.
